Wellbore profiling system

Abstract

A method is presented for accurately surveying and determining the profile of the path of a subterranean wellbore containing a constant density fluid extending contiguously throughout. A first pressure sensor, associated with a downhole tool, is traversed station-by-station along the wellbore for measuring the pressure of the fluid within the wellbore at each station. A second pressure sensor is located within the wellbore fluid at a known elevation. The elevation of the first pressure sensor, at a station, is determined by adding the calculated differential height to the known absolute elevation of the second sensor. As each elevation is referenced to the second sensor, no cumulative errors are incurred. If the density of the fluid is unknown, a third pressure sensor within the wellbore fluid can be provided at a known elevation different from that of the second sensor. The areal position of each station is determined by conventional means associated with the downhole tool. The elevation for each of a plurality of stations is combined with the areal position determined at each station to determine the path of the wellbore.

What is claimed is: 
     
       1. A method for determining the elevation at a survey point in a subterranean wellbore which is being drilled with a drill string which contains a continuous column of fluid having a known and substantially constant density, comprising: positioning a downhole tool in the drill string at the survey point, said tool carrying first means for measuring fluid pressure and being connected with second means for transmitting a signal which is indicative of the fluid pressure measurement to third means, located outside the wellbore, for calculating elevation of the survey point;   providing fourth means for measuring fluid pressure at a reference point of known elevation, said fourth means being in pressure sensing communication with the column of fluid and being connected with fifth means for transmitting a signal indicative of the fluid pressure measurement taken at the reference point, to the third means;   measuring the fluid pressure at the reference point and transmitting a signal indicative of the measurement to the third means;   measuring the fluid pressure at the survey point and transmitting a signal indicative of the measurement to the third means; and   calculating the elevation of the survey point by applying the third means which uses the pressure measurements, the density of the fluid and the known elevation of the reference point.   
     
     
       2. The method as recited in claim 1 wherein the density of the fluid is determined by: providing sixth means for measuring fluid pressure at a second reference point of known elevation different from the elevation of the forth means, said sixth means being in pressure sensing communication with the column of fluid and being connected with seventh means for transmitting a signal, indicative of the fluid pressure measurement taken at the second reference point, to the third means; and   measuring the fluid pressure at the second reference point and transmitting a signal indicative of the measurement to the third means; and   calculating the density of the fluid by applying the third means which uses the pressure measurements, the known elevations of the first and second reference points.   
     
     
       3. A method for determining the path of a wellbore having a bore containing a continuous column of fluid having a substantially constant density, comprising: (a) positioning a downhole tool at a survey point in the bore, said tool carrying means for measuring fluid pressure, means for measuring the traversed distance of the tool along the wellbore, and means for measuring the dip angle of the tool, all measured at the survey point;   (b) providing means for measuring fluid pressure at a reference point of known elevation along the length of the column of fluid;   (c) establishing measures indicative of the elevation of the tool at the survey point using the differential between the fluid pressure at the survey point and the reference point and the fluid density;   (d) establishing measures of the dip angle of the tool at the survey point;   (e) establishing measures of the traversed distance of the tool to the survey point;   (f) establishing measures of the horizontal location of the tool using the traversed distance and the orientation of the tool at the survey point   (g) moving the tool and measuring means to a new survey point; and   (h) repeating steps (c) through (g) for determining measures indicative of the profile of the path of the wellbore knowing the elevation, horizontal position and dip angle of the tool, where the azimuthal deviation of the path assumed to be zero.   
     
     
       4. The method as recited in claim 3 further comprising: providing means for measuring the azimuthal orientation of the tool at the survey point;   determining measures indicative of the departure of the survey point; and   determining measures indicative of the profile and plan of the path of the wellbore knowing the elevation, horizontal position, vertical and azimuthal orientation of the tool.   
     
     
       5. The method as recited in claim 4 wherein the azimuthal orientation measuring means are carried by the tool. 
     
     
       6. The method as recited in claim 3, further comprising: providing means for measuring fluid pressure at a second reference point located in the column of fluid and at a known elevation which is different than the first reference point; and   calculating the density of the fluid using the difference in fluid pressure pressures between the first and second reference points.   
     
     
       7. A method for controlling the direction of advance of a drilling string equipped with a bent sub and functioning to drill a horizontal wellbore, said string having a bore containing a continuous column of fluid having a substantially constant density, comprising: (a) positioning a downhole tool at a survey point in the bore, said tool carrying means for measuring fluid pressure, means for measuring the traversed distance of the tool along the wellbore, means for measuring the dip angle of the tool, means for measuring the tool's rotational orientation from vertical and means for measuring the bent sub's rotational orientation relative to the tool, all measured at the survey point;   (b) providing means for measuring fluid pressure at a reference point of known elevation along the length of the column of fluid;   (c) establishing measures indicative of the elevation of the tool at the survey point using the differential between the fluid pressure at the survey point and the reference point and the fluid density;   (d) establishing measures of the dip angle of the tool at the survey point;   (e) establishing measures of the traversed distance of the tool to the survey point;   (f) establishing measures of the horizontal location of the tool using the traversed distance and the orientation of the tool at the survey point   (g) moving the tool and measuring means to a new survey point; and   (h) repeating steps (b) through (f) for determining measures indicative of the profile of the path of the wellbore knowing the elevation, horizontal position and vertical orientation of the tool, where the azimuthal deviation of the path assumed to be zero, and for re-orienting the bent sub's rotation to change the direction of advance of the drilling string knowing the rotational orientation of the bent sub relative to the tool and the tool's rotational orientation from vertical.   
     
     
       8. The method as recited in claim 7 further comprising: providing means for measuring the azimuthal orientation of the tool at the survey point;   determining measures indicative of the departure of the survey point; and   determining measures indicative of the profile and plan of the path of the wellbore knowing the elevation, horizontal position, vertical and azimuthal orientation of the tool, and for re-orienting the bent sub's rotation to change the direction of drilling knowing the rotational orientation of the bent sub relative to the tool and the tool's rotational orientation from vertical.